1. Field of the Invention
The present invention relates to a method for forming high abrasion resisting layers on parent materials such as cams for driving valves of combustion engines.
2. Related Art
Cams for driving valves of an engine which rotate slidably contacting with locker arms and/or tappets are required to have a high abrasion resistivity. It has been known to harden the surface of the cam by remelting it using a heat source of a high energy density in order to enhance the abrasion resistivity.
When the surface of the cam of cast iron is remelted by the heat source having a high energy density, the portion remelted is rapidly cooled by the thermal conduction to the inner side of the cam to form a chilled layer having a high hardness and a fine structure and it becomes possible to obtain a hard layer of a high-alloy cast iron having a composition different from that of the parent material by supplying carbide forming elements such as Cr, Mo or the like upon remelting.
Conventionally, in the Japanese laid-open publication No. 60-230986, there is disclosed a method, as the surface hardening treatment for metal members such as cams, in which a sheet layer of an alloy power is formed on a surface of a parent metal member, for instance by adhering a sheet formed from a mixture of an alloy powder and an acrylic resin on the surface of the parent metal member, and after subjecting the sheet to a dewaxing treatment, the sheet layer of the alloy powder and the surface of the parent metal member are simultaneously remelted by a heating means having a high energy density to form a layer of a high alloy cast iron (See, for example, U.S. Pat. Nos. 4,608,225, 4,678,633 and 4,681,733).
As the heating means for remelting the sheet layer of the alloy powder, laser beam, electron beam, TIG arc (Tungsten Inert Gas arc) and the like are considered to be applicable. Among them, the TIG arc is very cheap when compared with others and, therefore, it is highly in demand to use the TIG arc for the remelting treatment.
However, there are some problems to be solved in the remelting method using the TIG arc.
Namely, in the alloy powder sheet to be remelted, grains or particles of a high melting point and grains or particles of a low melting point are coexisting in an inhomogeneous mixed state and grains or particles of a high melting point are apt to scatter upon remelting. As shown in FIG. 8 schematically, TIG arc torch 10 is positioned very near the alloy powder sheet 11 and, when the electrode 12 of the torch 11 is set within about 5 mm from the surface of the sheet 12, the electrode 12 is damaged severely by adhering scattered grains thereto.